In electrical supply networks, also described as electrical distribution networks, local network stations, or transformer substations, are used to transform and distribute electrical current. The local network stations or transformer substations that connect a medium voltage level and a low voltage level of a current network to one another can be constructed from a medium voltage switching system, a transformer and a low voltage distribution system. In order to locate short-circuits in the medium voltage network, short-circuit indicators are installed in the local network stations and the short-circuit indicators indicate any occurrence of an excess current.
A clearly directed load flow and radially operated networks led previously to a clear direction of the short-circuits in the energy supply networks so that a point of fault could be isolated using the short-circuit indicators. If medium voltage networks are operated in closed rings or rather as an intermeshed network (in other words, in this network, each network node is connected to one or more other network nodes), it is possible for occurring short-circuits to be supplied from different directions.
Furthermore, decentralized systems can lead to altered load flows and can likewise contribute to the excess current. In both cases, a mere identification of an excess current does not suffice for clearly isolating a point of fault. The network section that has the point of fault can however only be clearly located as a result of adding the direction information of the short-circuit or excess current. For this purpose, more recent short-circuit indicators use the capacitive pickup of the medium voltage switching unit that is provided in the proper sense only to establish an absence of voltage for a voltage measurement with a reduced measurement quality. If a capacitive pickup is not present, for example in air-insulated systems, it is not possible to identify the direction of the short-circuit or excess current.